1. Field of the Invention
The present invention relates to a circuit breaker, and particularly, to a trip mechanism for a molded case circuit breaker.
2. Background of the Invention
In general, a molded case circuit breaker is an electrical device for protecting an electric load or an electric circuit by breaking an electric circuit upon occurrence of fault current such as an over current, an instantaneous current and a short-circuit current between an electric power source and the electric load.
A molded case circuit breaker includes a terminal part by which a power source and an electric load are connected, a stationary (fixed) contactor electrically connected to the terminal part and disposed within the molded case circuit breaker, a movable contactor having a position contacted with the stationary contactor and a position separated from the stationary contactor, a switching mechanism for operating the movable contactor to contact the stationary contactor or be separated from the stationary contactor, and a trip mechanism for detecting occurrence of an overcurrent or an abnormal large current such as a short circuit current needed to be instantaneously broken, so as to trigger a trip operation of the switching mechanism for circuit breaking.
A representative example of the trip mechanism may be a full electromagnetic type trip mechanism using a thermal expansibility of oil within a cylinder wound by coil.
The construction and operation of the full electromagnetic trip mechanism according to the related art will now be described with reference to FIGS. 1 to 3.
FIG. 1 is a perspective view showing an inner construction of a molded case circuit breaker according to the related art, FIG. 2 is a perspective view independently showing a trip mechanism of FIG. 1, and FIG. 3 is a longitudinal sectional view of the trip mechanism of FIG. 2.
As shown in FIG. 1, a molded case circuit breaker 100 according to the related art includes an outer case 101 in which components are disposed, a switching mechanism 120 for rendering a movable contactor rotated to a position contactable with a corresponding stationary contactor (not shown) or a position separated from the stationary contactor so as to close (or turn on) a circuit or open (or turn off) the circuit, a trip mechanism 110 for detecting an occurrence of a fault current such as short circuit current on an electrical circuit so as to trigger a trip operation of the switching mechanism 120, an arc extinguishing mechanism 130 for extinguishing arc generated between the movable contactor 125 and the stationary contactor (not shown) during the trip operation, and terminals 140 by which an electrical power source or an electrical load of the circuit is connected.
The switching mechanism 120 includes a handle 123 for allowing a user to manually close or open a circuit of the molded case circuit breaker, a stationary contactor (not shown), a movable contactor 125 rotatable in response to the manipulation of the handle 123 and having a position contactable with the stationary contactor or a position separated from the stationary contactor, a crossbar (not shown) rotated by the trip mechanism 110, a latch holder (not shown) locked or released by virtue of the crossbar, and released upon the rotation of the crossbar so as to be rotated by a bias spring (not shown), a latch (not shown) having a position latched by the latch holder or a position released in response to the rotation of the latch holder, a trip spring (not shown) for providing the movable contactor with a driving force to rotate to the separated position upon release of the latch, a link (now shown) for transferring the driving force of the trip spring, and a shaft 124 connected to the link and for supporting the movable contactor 125 and rotating together with the movable contactor 125 so as to drive the movable contactor 125 to an open circuit position or a closed circuit position.
Hereinafter, detailed construction and operation of the trip mechanism 110 according to the related art will be described with reference to FIGS. 2 and 3.
As shown in FIGS. 2 and 3, the trip mechanism 110 includes a coil 111 connected to a circuit for generating an electromagnetic force upon occurrence of a fault current, such as a short circuit current on the circuit, a cylinder 113 inserted into the coil 111, a contactor 117 located at an upper end of the cylinder 113, an armature 115 abutted onto the contactor 117 by the electromagnetic force generated by the coil 111 so as to operate a trip lever 115a, and a frame 119 coupled with the cylinder 113 and the armature 115 and connecting a terminal part 140 to the coil 111.
Also, the cylinder 113 may be formed of a non-magnetic material, and provided therein with a plunger 114 movable by the magnetic force generated by the coil 111, and a plunger spring 114s for elastically supporting the plunger 114 such that an elastic restoring force can be applied in an opposite direction to a movement direction of the plunger 114. The cylinder 113 is sealed with containing oil 114a therein. Here, silicon oil is typically used as the oil 114a. 
Further, the armature 115 is elastically supported by an armature spring 115s so as to be restored to its original position after completion of a trip operation.
Hereinafter, the operation of the trip mechanism 110 according to the related art will be described with reference to FIGS. 2 and 3.
First, if an overcurrent larger than a rated current by about 120% is detected, a time-delay trip operation is normally performed. Under this condition, the plunger 114 exceeds the force of the plunger spring 114s by the magnetic force generated from the coil 111 due to the overcurrent, to thereby move toward the contactor 117, thereby attracting the armature 115. The movement of the attracted armature 115 then triggers the trip operation of the switching mechanism 120 shown in FIG. 1.
In the meantime, when a current several or several tens times larger than the rated current flows instantaneously, an instantaneous operation is performed. Under this condition, the magnetic force generated by the coil becomes drastically stronger than that in the time-delay operation. Accordingly, the armature 115 is attracted independently even if the plunger 114 moves or not, thereby triggering the trip operation of the switching mechanism 120 shown in FIG. 1.
Now, the triggered trip operation of the switching mechanism 120 will be described with reference to FIG. 1.
When the trip mechanism 110 detects a fault current such as a short circuit current on the circuit, the armature 115 provided at the trip mechanism 110 is rotated in a clockwise direction in FIG. 1 so as to push the cross bar. Accordingly, the crossbar is rotated and accordingly pushes the latch holder directly or via other components. The rotation of the latch holder releases the latch, and accordingly the trip spring discharges an accumulated elastic energy. The discharged elastic energy is transferred to the shaft 124 via the link (not shown). That the shaft 124 is then rotated in a clockwise direction in FIG. 1 and accordingly the movable contactor 125 supported by the shaft 124 is also rotated in the clockwise direction. As the movable contactor 125 rotated in the clockwise direction is separated from to the corresponding stationary contactor, the trip operation for breaking a circuit is completed.
However, the related art molded case circuit breaker 100 has several problems as follows.
First, because of the movement of the plunger 114 within the cylinder 113, the trip mechanism should be responsive to currents ranging from an overcurrent as a relatively low current compared to a short circuit current to an abnormal large current needed to be instantaneously broken, resulting in increasing a range of a magnetic force required for trip operation upon occurrence of the overcurrent and occurrence of the abnormal large current, such as a short circuit current needed to be instantaneously broken. To implement such wide range, the moving distance of the plunger 114 should be increased. Consequently, the cylinder 113 and the plunger 114 occupy a larger space and simultaneously increase the number of winding the coil 111.
Furthermore, viscosity of the oil 114a contained in the cylinder 113 changes depending on installation angles of the molded case circuit breaker 100, and the moving distance of the plunger 114 depends on the viscosity of the oil 114a. Consequently, upon occurrence of an overcurrent or an abnormal large current such as a short circuit current needed to be instantaneously broken, a characteristic of a trip operation problematically changes.